Solar/UV Info

[Cambium]

OK, so it is looking way out. The theoretical value is up around 240 at the end, but your station is only reading 115 W/m2

I see your station has a light sensor (lux) rather than a solar sensor, so Cumulus MX will be converting lux to W/m2.
Which station type do you use in Cumulus?

Interesting. I use HTTP (Ecowitt)

Here's latest...

SS.jpg

And in reality no clouds today so far. Full sun so about 5 hrs worth, meanwhile only 1.3 showing.

[mcrossley]

What you really need is a cloud free day to calibrate. If there are clouds around you will often get higher than normal readings at times - edge effect, and reflection. On a cloud free day you should get a nice bell curve from your station that matches the theoretical. If it is offset in time (and your station longitude in MX is accurately set) it indicates a tilt in your sensor that needs fixing.

When you get a curve like that, examine the maximum value from your station, add 5 to 10%. Then the following day around midday adjust the solar RS transmission factor for both June and December so that the theoretical value comes out about that number you calculated. It's only calculated once a minute so be patient. It's an iterative process I'm afraid.

Then around the summer solstice, repeat the exercise, but just adjust the June value.

[Cambium]

What you really need is a cloud free day to calibrate. If there are clouds around you will often get higher than normal readings at times - edge effect, and reflection. On a cloud free day you should get a nice bell curve from your station that matches the theoretical. If it is offset in time (and your station longitude in MX is accurately set) it indicates a tilt in your sensor that needs fixing.

When you get a curve like that, examine the maximum value from your station, add 5 to 10%. Then the following day around midday adjust the solar RS transmission factor for both June and December so that the theoretical value comes out about that number you calculated. It's only calculated once a minute so be patient. It's an iterative process I'm afraid.

Then around the summer solstice, repeat the exercise, but just adjust the June value.

Thanks Mark excellent info and advice. I will do that.

I'm wondering if some trees/branches is the reason it's not a nice bell curve, I'll have to keep a better eye on that but will look into fixing the sensor if its that.

Today was cloud free. Blue skies all day. As of 4pm I counted 8.5hrs of sunshine in reality but its showing 4.6.

SS1.jpg

[Gyvate]

1. trees, bushes etc. can reduce the solar irradiation
2. even blue sky may not be without obstacles which attenuate the irradation e.g. areas of water steam (invisible but radiation absorbing)
3. some clouds can create the well-known cloud-edge effect which can create irregulaties and also readings higher than the theoretical maximum
even water droplets on the light sensor can have such effects - it's not a simple and ideal scenario when you look into the matter more closely.
4. there remains a difference between what the human eye physiologically experiences as sunshine and the numeric energy value or the luminosity measured by the sensors. That's why these formulas are approximations which work best when there is blue sky and there are no obstacles (buildings, trees, hills, mountains ....).
5. Some sunlight may fall on you and your eye(s) and you will call it sunshine, but it may not hit the sensor or only indirectly through reflection etc.

So it's not a homogeneous situation which creates what your console receives from the light sensor.

Given all these factors, I was happy to have found this Blake-Larsen sunrecorder which takes such situations - at least better than just a formula - into account by imitating the human eye; but it isn't perfect either, especially in winter periods with low sun declination and therefore also bigger exposure to obstacles. Of course it (the sunrecorder) also needs to be properly positioned - sufficiently high and with the proper orientation (azimut and declination depending on latitude). But also the way it is constructed and collects sunlight is very different from the comparatively primitive light sensors most personal weather stations have.
In my personal experience the B-L SR readings come pretty close to what I experience as times of sunshine.

[SamiS]

I'm wondering if some trees/branches is the reason it's not a nice bell curve, I'll have to keep a better eye on that but will look into fixing the sensor if its that.

Today was cloud free. Blue skies all day. As of 4pm I counted 8.5hrs of sunshine in reality but its showing 4.6.

All the graphs you have posted so far do seem to have the same ”feature”, that at morning from about 8 to 11 the values are low, and then approximately at midday it jumps to the calculated maximum or a bit above it. So I would say that there is definitely something blocking sunlight from the sensor. If it is not trees or branches, maybe dirt on the sensor itself.

[mcrossley]

Here is an example of mine (before I moved it to remove the bit of shadowing morning and evening.
.

perfect_day.png

As you can see, there is still a bit of variation even on what appeared to be a perfectly clear day. We are near a flight path so even diffuse contrails have an effect.

Just out of interest, here is another day, not plotted in MX, but when I was comparing three different theoretical models against my sensor.
.

Capture1.PNG

[Cambium]

Awesome discussion guys. Mark, That graph is so smooth! I have to play around with the numbers.

I just went up there and took this pic. I had a feeling but now I realize the reason its erratic in the morning..



Closeup of the sensor

[SamiS]

It seems your sensor is the older version (clear plastic over the sensor), so it might be one reason why it is quite sensitive to any obstacles. Later versions have white milky lens that probably softens and distributes the light getting to the sensor.

[Cambium]

Leaving this here for myself to keep note of.. Lucky to have a stretch of blue skies not 1 cloud.

I'm using this Bash Emoji because I just realized I was changing the June Transmission instead of December.

Can't do much about that morning tree interference but at least it's picking up over 8 hours of sunshine now vs only 5 on the 3rd.

Feb 4th


Feb 5th


Feb 6th

[Cambium]

Question... Need advice again.. Sorry.

So I got clouds all morning with a dim poke here and there. And it's reading 1.2hrs of sun today. If the Theoretical Max at 10:30am was 388 adding 60% to that would mean the sun is shining at 620 W/m2? Is that correct? If so why is it detecting sunshine?

Which setting should I adjust?

Sun9.jpg

[mcrossley]

From the graphs above your theoretical value looks like it is way below your sensor reading**, so you need to bump that up significantly. Bring the both transmission factors up until you are getting a theoretical value around 550(ish) at mid-day. Then worry about the threshold - leave it around 75% for now.

** Or put another way your sensor looks like it reads high!

[Cambium]

From the graphs above your theoretical value looks like it is way below your sensor reading**, so you need to bump that up significantly. Bring the both transmission factors up until you are getting a theoretical value around 550(ish) at mid-day. Then worry about the threshold - leave it around 75% for now.

** Or put another way your sensor looks like it reads high!

Thanks Mark! Done. Bumped it to 0.8. I'll see how this works out. I got the peak at 529 midday for now. I'm worried if its too high it doesn't pick up the morning sunshine through the bare trees. So far with the new adjustment it picked up an extra hour of sun from 11:07-1pm which it has been shining

Is it correct to say the 65% I have means the sun is shining 65% above where the theoretical value is for that minute? Can't be; looking at the graph and the fact that it added another hour of sunshine

Sun11.jpg

[mcrossley]

Is it correct to say the 65% I have means the sun is shining 65% above where the theoretical value is for that minute?

No, it will count sunshine when the station value at or above 65% of the theoretical value.

[Cambium]

Is it correct to say the 65% I have means the sun is shining 65% above where the theoretical value is for that minute?

No, it will count sunshine when the station value at or above 65% of the theoretical value.

Ohhhh! Thank You!

An idea for you is to add to CMX a line on that solar graph representing the threshold. Would be cool to see the actual line of where and when the sun is shining based off the minutes

[mcrossley]

A bit like this? https://weather.wilmslowastro.com/graph ... .php#solar

(Switch on Solar %)